1. Field of the Invention
The present invention relates to an image pickup apparatus for picking up an object image and a drive method therefor.
2. Related Background Art
As typical solid-state image pickup element, an element formed from a photodiode and CCD shift register and an element called an APS (Active Pixel Sensor) formed from a photodiode and MOS transistor are known.
An APS includes for each pixel, a photodiode, MOS switch, amplification circuit for amplifying a signal from the photodiode, and has may technological advantages that XY addressing is possible, and a sensor and signal processing circuit can be formed on one chip, for example. However, since many elements are included in one pixel, the chip size that determines the scale of the optical system can hardly be reduced, so CCDs are currently most popular. An APS has received a great deal of attention because the MOS transistor micropatterning technique is improved, and strong demands have arisen to “make a sensor and signal circuit on one chip” or to “reduce power consumption”.
FIG. 13 is an equivalent circuit diagram of an APS. Referring to FIG. 13, a transfer switch Q1 transfers photocharges from a photodiode 101 to a floating diffusion area (FD). A reset switch Q2 resets the floating diffusion area. An input MOS transistor Q3 is included in a source follower for outputting the voltage in the floating diffusion area. A selection switch Q4 selects a pixel. The APS also includes a power supply line 102, reset switch control line 103, transfer switch control line 104, selection switch control line 105, and signal output line 106. The operation of the APS using a scheme, in which the selection switch control line 105 for controlling the selection switch Q4 is commonly arranged in the row direction so as to select a row and transfer charges of one row to a line memory at a time, will be described.
FIG. 14 shows a drive pulse used to read out a pixel (row).
Before the start of accumulation operation, while keeping the reset switch Q2 turned on, the transfer switch Q1 is turned on to temporarily reset the photodiode 101, as shown in FIG. 14. The floating diffusion area is set in the floating state, and the selection switch for reading-out is turned on. A voltage corresponding to the voltage in the floating diffusion area is output to the signal output line 106 by a source follower formed from a load connected to the input MOS transistor Q3 and signal output line 106. This output is sampled into a memory. That is, reset noise is sampled. After that, to transfer the photo-signal charges to the floating diffusion area (FD), the transfer switch is turned on and off. Transfer is done while keeping the floating diffusion area in the floating state. For this reason, the voltage in the floating diffusion area is a voltage lower than a voltage Vres immediately after the reset, by Q/CFD (Q is transferred charges, and CFD is the capacitance of the FD portion), which is given byVFD=Vres−Q/CFD That is, Q/CFD is superposed on the reset voltage containing different reset noise components at each reset. Since a signal corresponding to the voltage is output to the signal output line 106, this signal is sampled (“photo-signal plus sampling of reset noise” in FIG. 14).
Finally, the “reset noise” signal and “photo-signal+reset noise” signal are subtracted from the photo-signal by a differentiating circuit whereby the reset noise which is reset to different voltages at each reset can be removed.
Especially, when a buried photodiode is used as the photodiode 101 in FIG. 13, the reset noise can be completely removed, and a high S/N ratio can be obtained.
However, recent APSs have problems of a residual image and random noise. The reasons for this will be described below in detail.
As described above, recent APSs use micropatterned MOS transistors, thereby realizing a small pixel size. For this reason, a residual image or random noise due to new reasons based on the micropatterned MOS transistors occurs.
FIG. 15 is a sectional view of an APS using a micropatterned MOS transistor. FIG. 16 is a potential chart corresponding to the sectional view of FIG. 16. As shown in FIG. 15, a surface p-type area 304 is formed on an n-type area 305 of a photodiode (called a buried photodiode). Charges accumulated in the n-type area 305 are completely transferred to a floating diffusion area 303 through a transfer switch (SW) 302 by a depletion layer extending from a PWL (p-well) 301 and surface p-type area 304. The floating diffusion area 303 is connected to a reset electrode 307 through a reset switch (SW) and can be set to a predetermined reset potential.
In the APS using a micropatterned MOS transistor, a potential pocket and barrier are formed, as indicated by A and B in FIG. 16. As a result, some of charges in the transfer switch area return to the photodiode to generate a residual image or random noise.
The pocket (A in FIG. 16) and barrier (B in FIG. 16) of the potential will be described in more detail.
To form a micropatterned MOS transistor, annealing in the manufacturing process must be executed at a lower temperature in a shorter time. Hence, an impurity, such as boron or phosphorus, for determining the conductivity type is diffused not only simply by heat but also through defects and the like, i.e., the influence of transient enhanced diffusion phenomenon becomes conspicuous. As a result, boron for determining the p-type well region segregates to an area corresponding to B in FIG. 16, thereby forming a potential barrier. This phenomenon is known as a reverse short channel effect in a micropatterned MOS transistor.
Such a phenomenon typically occurs in an area having a channel length L of 1 μm or less and, more particularly, a channel length L equal to or smaller than 0.7 μm.
On the other hand, an area corresponding to A in FIG. 16, i.e., a portion where the n-type area 305 of the photodiode 101 connects to the transfer switch 302 determines the signal charge transfer characteristic. The positional relationship between the surface p-type area 304 and the n-type area 305 of the photodiode is very important. Especially when a micropatterned MOS transistor is used, the voltage to be applied to the gate of the transfer switch 302 must be reduced. In this case, however, the potential of the transfer switch cannot be sufficiently dropped, and the charges are difficult to be transferred. In some cases, to promote charge transfer, an n-type bypass area 308 is positively formed, as shown in FIG. 15.
The width of such a bypass area 308 is 0 to 0.5 μm. If the bypass area 308 is too narrow, transfer is difficult. If the transfer width is too large, a potential pocket is formed. When the voltage applied to the gate of the transfer switch 302 is high, the voltage can compensate for transfer, and be designed as a value that prevents any potential pocket. However, when a micropatterned MOS transistor is used, the voltage cannot compensate for transfer, and the width controllability is as strict as 0.05 μm or less, i.e., a stricter controllability than that for the gate length of a micropatterned MOS transistor is required. As a consequence, a potential pocket is readily formed.
In the conventional art, charges remain under the gate of the transfer switch 302 due to the above-described potential pocket (A in FIG. 16) or potential barrier (B in FIG. 16).
The remaining amount is given byRemaining amount ∝ (VTXH−VTXth−VFD)where VTHX is the high-level gate voltage of the transfer switch 302, VTXth is the threshold voltage of the transfer switch 302, and VFD is the voltage in the floating diffusion area.
When some or all of the residual charges return to the photodiode, a residual image is generated. Depending on the operation condition, residual charges are generated even in a dark state and return to the photodiode. The residual charges thermally fluctuate to make random noise.
FIGS. 17A to 17D show potential states when the transfer switch is turned on and off. FIG. 17A is a potential chart showing a state immediately after the floating diffusion area is reset. Signal charges are accumulated in the photodiode. FIG. 17B is a potential chart showing a state wherein the transfer switch is turned on to transfer the signal charges to the floating diffusion area. At this time, the potential of the floating diffusion area is increased by the signal charges.
Since the surface potential under the gate of the transfer switch becomes lower than the potential in the floating diffusion area depending on the amount of signal charges, charges are generated even under the gate of the transfer switch. FIG. 17C shows a state wherein the transfer switch is turned off, i.e., a state wherein charges induced under the gate of the transfer switch cannot completely move to the floating diffusion area. The charges that cannot completely move return to the photodiode side to cause a residual image or random noise.
In a CCD, the power supply voltage is high. For this reason, even when maximum signal charges are transferred, the surface potential of the transfer switch is always higher than the vertical CCD potential, and no charges are generated under the gate of the transfer switch.
In the APS using a micropatterned MOS transistor, the power supply voltage is low. For this reason, when at least maximum signal charges are transferred, the state shown in FIG. 17B occurs, and a potential relationship as in a CCD can hardly be formed. In addition, when the reset voltage is low, the state shown in FIG. 17B occurs even in a dark state, resulting in random noise in the dark state.